Catalytic process and apparatus



Jan. 28, 1947. H. w. BURNSIDE ErAL 2,

cn'rgunc *rnoc:-:S AND APPARATUS Filed April 11, 1944 2 Sheets-Sheet 114 1D FRACTIONATING EQUIPMENT Hamel E. W. bumside Henry J. OqorzalqINVENTUQS Jan. 28, 1947. H. E. w. B URNSIDE EI'AL- 2,414,352

CATALYTIC PRQCESS AND APPARATUS Filed April 11, 1944 2 Sheets-Sheet 2 &

Harveq EM. burrgsidq INVENTORS Heart; J.

Patented Jan. 28, 1947 v 2,414,852 UNITED STATES. PATENT orrlcsCATALYTIC PROCESS AND APPARATUS Harvey E. W. Burnside and Henry J.Og'orzaly,

Elizabeth, N. 1., assignors to Standard Oil Development Company, acorporation of Delaware Application April 11, 1944, Serial No. 530,450

. 10 Claims.

This invention relates to catalytic processes and pertains moreparticularly to a catalytic process in which a finely divided catalystis caused to circulate continuously through a treating zone.

While the invention in its broader aspects has a more generalapplication, it is especially adapted to processes and apparatus for thecatalytic cracking of hydrocarbon oil wherein the catalyst iscontinuously circulated through a reaction zone and a regenerating zone.

One method now employed for cracking hydrocarbon oil to form motor fuelis to crack the oil in the presence of a finely divided crackingcatalyst which is continuously circulated through the cracking andregenerating zones by the pressure generated by a column or a pluralityof columns of catalytic material in which the catalyst is maintained ina relatively dense, fluid state similar to aliquid and capable ofdeveloping ahydrostatic pressure.

According to present practices, the rate of.circulation of the powderthrough the cracking and regenerating zones is regulated by controlvalves usually positioned at the base of-the column or standpipe. Inorder to permit substantial variations in the rate of flow and alsoprovide an effective seal between the cracking and regenerating zones,the apparatus is normally designed to provide from 2 to 5 poundspressure drop across the valves and in some operations an even greaterpressure drop is desirable.

One of the objects to provide a method of the general character aboveoutlined wherein the height of the columns or standpipe may be reduced.

It is also a practice, when carrying out a cata lytic process as aboveoutlined, to pass the oil vapors upwardl through the cracking zone at avelocity controlled to maintain a relatively dense fluidized layer ofcatalytic material in the bottom portion of the cracking or regeneratingzone. It has been found, for example, that under properly controlledconditions the catalyst remains largely segregated in a relatively denselayer at the bottom of the cracking or regenerating zone and this layeris superimposed by a free space wherein the gases being treated containonly a relatively small amount 'of entrained catalyst particles.

A broader object of the present invention is to provide an improvedprocess for circulating a powder through a treating zone.

A more specific object of the invention is to for the catalyticconversion of hydrocarbon oils.

Another object of our invention is to provide an improved apparatus forcirculating catalytic or other finely divided contact materials througha treating zone, Other objects and advantages of the invention will beapparent from the de- -in the form of an enlarged vertical reactionvessel. For illustrative purposes, the invention will be described asapplied to the catalytic cracking of hydrocarbon oils, it beingunderstood that in its broader phases the inventionmay have a moregeneral application as previously pointed out.

The oil to be cracked in admixture with a finely divided crackingcatalyst is introduced into the bottom section of chamber In throughline 1 ll. Any type of solid material capable of exofthe'"presentinvention is erting the desired catalytic effect on the oil vapors maybe employed in the operation. Par-,- ticularly suitable catalystscomprise acid-treated bentonite clays or synthetic siliceous gels oi thesame or different composition. The catalyst is preferably in finelydivided form, having a particle size generally smaller than 200 mesh.

The bottom portion of the chamber 10 may take the form of an invertedcone forming a. distribution zone for distribution of the oil andcatalyst over the full cross-sectional area of the reactor. It is alsopreferable to provide a per forated grid in the bottom section of, thereactor for insuring more thorough distribution of the provide a moresimple and inexpensive method I catalyst andoil vapors therein.

The velocity of the oil vapors passing upwardly through the reactor I0is preferably controlled to permit the catalyst to segregate into arelatively dense layer inthe bottom section of the chamber asillustrated. When employing catalytic materials of the type abovementioned, a velocity of the order of 0.5- to 5 feet per second may beemployed. It will be-understood that the velocities here mentionedhave-reference to the normal linear velocities which the oil vaporswould attain in the absence of the solid material within the reactor.

It is preferable to construct the reactor ll) of suflicient height toprovide a substantial free space above the level ofcatalytic material inthe reactor Ill so as to reduce the amount of--catalyst entrained in thegases leaving the reactor, For this purpose a free space of from 4 to 10or more feet is desirable. I

The reactor I0 is maintatined under cracking temperatures ranging from750 F. to 1000 F. or more. This temperature may be maintained by theintroduction of hot catalytic material into the oil vapors as laterdescribed.

The cracked vapors, after passing through the reaction zone, may bepassed through a suitable regenerator l'l. wardly through the gridintermixes with the separating device such as a cyclone separator l2 forremoving entrained catalytic material therefrom. The catalyst separatedfrom the cracked vapors in the cyclone separator I2 is returned to thereactor through tube l3 below the level of the dens fluidized mass ofcatalytic material therein. The cracked vapors after passin through thecyclone separator I2 are withdrawn from the reactor l through-line l4and may be passed to suitable fractionating equipment which, forsimplicity, has not been shown on the drawings.

The reaction chamber I0 is also provided with 9. depending conduit 15having a portion thereof extending into the main body of the reactionchamber for continuously withdrawing a portion of the catalytic materialtherefrom. The conduit I5 discharges through a tube l6 extending intothe regenerator ll. The tube It projects into the regenerator ll to apoint below the level of a dense fluidized layer of catalytic materialundergoing regeneration therein. The conduit l5 and the tube 16 form astandpipe or column positioned to build up or generate a pressure equalto the pressure difierence'between the reactionand the regeneratingzone,

A stripping gas such as steam, spent combustion gases or the like may beintroduced into the bottom portion of the conduit l5 to removevaporizable hydrocarbon constituents from the catalyst passing into theregenerating zone.

It is also'desirable to maintain the catalytic material in the conduitand the tube It in a freely flowing, fluidized state to prevent packing0r bridging of the catalyst therein and to this end a small amount of anaerating or fluidizing gas may be introduced at one or more spacedpoints into the conduit l5 and the pipe IS. The amount of fluidizing gasso introduced is relatively small and only suilicient to form a thinfilm of gas around the individual particles within the conduit.

The regeneration chamber I1 is constructed in the same general manner asthe reaction chamber Ill, The air for regenerating the catalyst byburning up the combustible deposits therefrom is introduced into thebottom section of the regenerator I! through line 18. Regenerator l'lmay be provided with an inverted conical bottom forming a distributingzone for distributing gas into the regenerating chamber,

The regenerating chamber may also b vided with a perforated grid in thebottom portion thereof through which the gas or other oxidizing mediumpasses into the main body of the Th oxidizing gas passing upflnelydivided spent catalytic material introduced into the regenerator I]through line l6 and serves to burn the combustible deposits therefrom.

As previously described with reference to the reaction chamber III, thevelocity of the oxidizing gas passing upwardly through the regeneratorI1 is preferably controlled to maintain the catalytic material withinthe regenerator in a relatively dense, turbulent layer in the bottomsection thereof. This velocity may be of the order of from 0.5 to 5 feetper second.

The spent combustion gases after passing through the layer of catalyticmaterial in the bottom portion of regenerator I! may be passed throughsuitable separating device such as oyclone separator l9 inwhichcatalytic material entrained in the gases may be removed. The catalystseparated from the regenerating gas is 4 returned to the bottom portionof the regenerator l1 through tube 2 I.

The spent combustion gas is removed from the regenerator I! through line22, having a pressure control valve or other means of pressure con-'trol 23 actuated as later described to maintain a predetermined pressuredifference between the reaction chamber and the regeneration chamber. Astream of regenerated catalytic material is continuously withdrawn fromthe regenerating chamber ll through the conduit 24 which extendsupwardly into the body of the reactor similar to the conduit l5 aspreviously described.

The conduit 24 is provided with a suitable control valve such as theslide valve 25 for regulating the amount of catalytic material withdrawnfrom the regenerator. The stripping or fluidizing gas is introduced intothe conduit 24 at one or more spaced points to maintain the catalyticmaterial in a fluid, liquid-like state. In some cases it may bedesirable to provide cooling tubes or other cooling elements in theconduit 24 in order to extract excess heat liberated during regenerationof the catalyst.

The regenerated catalytic material is discharged from the conduit 24into a stream of oil passing through line H and is returned to thereaction chamber as previously described. In order to insure continuouscirculation of the catalytic material through the reacting andregenerating zone as previously described, it is important to maintainthe finely divided catalyst in a freely flowing, fluidized state. Tothis end, a small amount of an aerating or fluidizing gas should beinjected into the catalyst at any point in the circuit where there is atendency of the catalyst to separate into a dense, compact mass.

Under these conditions the catalytic material may be caused to flow as aliquid and the laws of fluid flow may be applied. With this in mind, itwill be noted that in order for the catalyst to flow from the reactorinto the regenerator as described, the back pressure on the regenerationgas at the top of the regenerator must be less than the inlet pressureon the catalyst passing into the regenerator through the tube orstandpipe It. On the other hand, in order to maintain a level ofcatalyst in the reactor Ill above the conduit l5,

it is necessary that the back pressure on the regeneration gas must begreater than the back pressure on the oil vapors above the catalystlayer in the reactor l0.

Within these limits, the level of catalyst within the reactor ID will bedetermined by the adjustment of the pressure control valve 23. Atequilibrium conditions, the difierence between the outlet pressure onthe regenerator and the outlet pressure on the oil vapors in the reactorI 0 will be equal to the hydrostatic or fluid pressure generated by thestandpipe or column I6 and the superimposed layer of catalytic materiallocated in the reactor l0 above the drawoif pipe 15. This fluid pressurewill vary with the level of the catalyst in the reactor H).

In accordance with the present invention, the difference between thepressure on the gases in the free space above the catalyst layer in theregenerating chamber IT and the pressure on the vapors in the free spaceabove the catalyst level in the reactor I0 is used for regulating thepressure control valve 23. For example, a decrease in the pressuredifference between the gases and v 5 be corrected and the catalyst layerrestored to the desired level by partially closing the throttle valve orpressure controller in line 22, thus increasing the back pressure on theregenerator and there- 'by reducing the rate of flow from the reactor tothe regenerator until the normal level has been restored. Conversely, anincrease in the pressur difference between the gases and vapors in theregenerator and reactor, respectively, indicates an increase in thedepth of the catalyst layer in the reactor I 0. This, can be correctedby turning the throttle valve 23 toward the open position, therebyreducing the amount of back pressure causing an increase in the rate offlow through the standpipe I6 so as to restore the level to its originalposition.

The level of the catalyst layer within the reactor H] can therefore beregulated within desired limits by operating the throttle valve 23 inresponse to variations in the difference between the pressure on thegases in the free space above the catalyst layer in the regenerator andthe pressure on the oil vapors in the free space above the catalystlayer in reactor l0. While the throttle valve 23 may be controlledmanually as above outlined, it is preferred to operate the'valve by anautomatic diiierential pressure controller. One simple and effective wayis illustrated in the drawings wherein the upper end of the regeneratorI1 is inopen communication, through line 21, with one end 28 of aU-shaped manometer .29 and the upper end of the reactor I is in opencommunication through line 3| with the other leg 32.

The difference in level'of the mercury in the two legs shows thepressure difierence between the two chambers.

The U-shaped manometer may be provided with suitable electrical contactsso that any predetermined fluctuations in the level of the mercurytherein will cause closing of an electric circult in the operation ofthe control valve 23. As indicated, the level of mercury in the leg 28will normally be lower than the level in the leg 32 due to the higherpressure in the regenerating zone. The difference in the level of themercury in the two legs indicates the amount of hydrostatic pressuregenerated by the column of fluidized catalyst between the upper level ofthe catalyst in the reactor II] and the upper level of the catalyst inthe regenerator l1, and. the amount of this hydrostatic pressure sodeveloped depends upon the level maintained in the reactor l0.

By properly positioning the electrical contact in the U-shapedmanometer, the level of catalyst in reactor Ill may be controlled withinthe desired limits.

When operating in accordance with the present invention, the valve 25 inthe conduit 24 controls the rate of circulation of the catalyst throughthe cracking and regenerating zones and the throttle valve 23 in theoutlet of the regenerating chamber controls the level of catalyst in thereactor l0.

In practice, it is desirable to position the reactor a sufficientdistance above the regenerator to build upa differential pressurebetween the two chambers of from 3 to 10 or more pounds per square inch.When employing conventional catalysts having a fluid density of about 30pounds per cubic foot, about 5 feet of catalyst height is required foreach pound of. pressure developed. In some cases, it may be desirable toprovide a safety valve in the conduit l6 which can be operated in caseof any abnormal fluctuation in pressure within the circuit.

According to another phase of the invention, the velocity of the airpassing upwardly through the regeneration chamber I1 is controlled so asto prevent overall recirculation of catalyst within theregenerator I1 sothat the catalyst flows downwardly through the regenerator in a generaldirection countercurrent to the flow of the gases passing upwardlytherethrough. In order to accomplish this, the velocity of the uprisingvapors may be of the order of from 0.1 to 0.5 feet per second, dependingupon the size of the catalyst particles and other factors. Under theseconditions the concentration of carbon on the catalyst in the upperportion of the regenerator will be greater than the concentration at thebottom of the regenerator from which the regenerated cataserve as astripping agent for reducing the amount of carbon to be burned. A partof the spent regeneration gas withdrawn from the regenerator ll may bepassed through line 33 and injected into the conduit l5 as a strippingmedium therefor or a separate stripping as may be introduced throughline 34.

7 While in the previous description the reactor has been located at ahigher level than the regenerator, in many cases it may bedesirable toposition the reactor at a lower level. For example, it is desirable tomaintain a back pressure on the reactor in order to pass the oil vaporsthrough the subsequent fractionating and recovery equipment and theregenerator may be operated at substantially atmospheric pressure. Thelower vessel I'l may be utilized as a reactor by introducing oil intoline [8 instead of air and introducing air through line I I instead ofoil and connecting line 22 with the fractionating equipment rather thanline l4.

The invention-finds application in processes in which it is unnecessaryto regenerate the finely divided material, but in which it is desirableto pump or otherwise continuously circulate finely divided powderedmaterial through the treating zone. In such cases the upper chamber Illillustrated in Fig. I mayserve. simply as a separating device forseparating the powder from the gas stream passing through line H so thatit may be returned to the reaction chamberlocated at the lower level.

Fig. 11 illustrates a simplified form of equipment in which a powder iscontinuously circulated through a reactor and in which the principles ofthe invention may be utilized,

Referring to Fig. II, the reference character 40 designates a reactionchamber constructed similar to the reactor [0 or regenerator l1 shown inFig. I. The gases to be reacted 'are introduced into the reactor 40through line 4| and pass upwardly into the main portion of the chamberrator or other suitable separating device 43 and thereafter removed fromthe reactor through line 44. The powder separated in the separator 43returns to the reactor through conduit 45. A stream of finely dividedpowder is continuously withdrawn from the reactor 40 through fve'rticalstandpipe or conduit 46 into which a small amount oi fluidizing gas maybe introduced at. one or more points through lines 41 and 48 Theconduit' 46 isprovided with a control valve 49 through which the powderis withdrawn from the chamber at a controlled rate and discharged into astream of gases passing through line This stream of gases may be passedupwardly through lines 52 and 53 into the bottom of a separator 54having a construction similar to the reaction chamber 40. The velocityof the gases passing upwardly through the separator is controlled topermit primary separation of the powder from the gas stream to form arelatively dense layer of solid material therein. This layer providesprimary separation of the powder from the gases introduced into theseparator. The gases may then pass to a second or final separatingdevice such as a cyclone separator 55 positioned at the top for removingentrained powder and the gases finally vented through line 55.

The separator 54 discharges through the vertical standpipe 55 directlyinto the reaction chamber 40. If desired, a fluidizing gas may beintroduced into the standpipe 55 through one or more pipes 51 and 58.Instead of introducing the suspension of powder and gases into thebottom section of the separator 54, the suspension may be passed throughline 59 into the cyclone separator 55 and the bulk of the materialseparated by this separator, In the latter case, the powder-separatedfrom the gas stream drops into the bottom portion of the separator 54which serves as a hopper or reservoir from which the powder continuouslydrops through theconduit 55 into the reactor 40. As already described inconnection with Fig. I, the outlet line 44 from the reactor 40 isprovided with a differential pressure control valve or other means ofpressure control 62 for regulating the back pressure on the reactor 40and this control valve is preferably regulated by the differentialpressure between the vapor space above the layer of powder in theseparator 54 and the vapor space above the layer of powder in thereactor 40. 'This pressure control valve tends to maintain a definitelevel of powder in the separator 54 and maintains a seal for preventingthe reaction gases from by-passing upwardly through the conduit 56.

The apparatus illustrated in Fig. II is particularly suitable where itis desirable to circulate powder through a reaction chamber for thepurpose of regulating the temperature therein. For example, in reactionsinvolving strong heat effects, such as high exothermic or highlyendothermic reactions, it may be desirable to extract heat from thereactor by circulating the powder through the reactor and thereafterthrough a heat exchanger for adding or extracting heat from the powderundergoing circulation. The heat may be extracted or added to the powderbeing circulated by means of a heat exchanger in the separator 54 shownin Fig. II, or a suitable heat exchanger may be placed in either theline 53 or line 59. A heat exchanger 63 has been shown in the line 59,it being understood that a similar heat exchanger may be provided inline 53 in cases where the hopper 54 serves as a primary separator aspreviously described.

Having described the preferred embodiment of the int ention, it will beunderstood that it embraces such other variations and modifications ascome within the spirit and scope thereof.

We claim:

1. A method of. circulating a finely divided solid material through atreating zone which comprises maintaining a body of said finely dividedmaterial within said treating zone, passing a gas upwardly through saidtreatin zone, continuously withdrawing a stream of finely dividedmaterial from the bottom portion of said treating zone, introducing thefinely divided material so withdrawn into a second stream of gas,transferring said last-named gas stream containing said finely dividedmaterial to a separating zone at an elevation substantially higher thansaid treating zone, separatingthe finely divided solid material fromsaid'last-named gas stream, transferring the finely divided solid soseparated into the top of a vertical column of said finely divided solidmaterial, maintaining a fluidizing gas in admixture with said finelydivided solid material within said column in an amount controlled tomaintain said material in a fluid state capable of generating a fluidpressure, discharging finely divided material from the base of saidcolumn into said treating zone by the fluid pressure generated by saidcolumn, determining the pressure difference between said separating zoneand said treating zone and controlling the back pressure on saidtreating zone in response to said pressure difference to regulate therate of feed of said finely divided solid material into said treatingzone.

2. A process for continuously circulating a finely divided solidmaterial through a plurality of treatin zones which comprisesmaintaining a relatively dense fluidized layer of catalytic material inone of said treating zones, maintaining a second layer of finely dividedsolid material. in a second treating zone at an elevation substantiallyhigher than said first-named treating zone, maintaining a confinedcolumn of said finely divided solid material between said treatingzones, the upper end of said column being in open communication withsaid layer of material in said upper treating zone and the lower end ofsaid column being in open communication with said layer in said lowertreating zone, maintaining an aerating gas in admixture with said solidswithin said column in an amount controlled to maintain said column in afluidized state capable of generating a fiuid pressure, passing a gas tobe treated upwardly through said lower treating zone, continuouslydownwardly withdrawing finely divided material from said lower treatingzone at a controlled rate, discharging the finely divided material sowithdrawn into a stream of gases, passing said last-named stream of gascontaining said finely divided solid material to said upper treatingzone, and regulating the back pressure on said lower treating zone tocontrol the rate of flow of said finely divided material from said uppertreatin zone and through said column and maintaining the desired levelin said upper treating zone.

3. In the process defined by claim 2, the further improvement whichcomprises controlling the back pressure on the lower treating zone inresponse to the pressure difference between the upper treating zone andthe lower treating zone.

4. A process for the catalytic conversion of hydrocarbon oils in whichfinely divided catalyst is caused to circulate continuously through aconversion zone and a regenerating zone which comprises maintaining arelatively dense fluidized layer of finely divided conversion catalystwithin a converison zone, passing hydrocarbons to be converted upwardlythrough said conversion zone at a rate controlled to prevent substantialencommunication with the zone of lower elevation, 1

maintaining the finely divided conversion catalyst in a freely flowing,fluidized state capable of generating a fluid pressure within saidcolumn, continuously downwardly withdrawing a stream of finely dividedconversion catalyst from the lower of said zones, discharging the streamof catalyst so withdrawn at a controlled rate into the stream of gasespassing to said upper zone and controlling the back pressure on thelower treating zone to regulate the level'of finely divided conversioncatalyst in the upper treating zone.

5. In the process defined by claim'4, the further improvement whichcomprises positioning the conversion zone at an elevation above theregenerating zone.

'6. The invention defined by claim 4 wherein the regenerating zone ispositioned above the conversion zone. 1

7. A process for the catalytic cracking of hydrocarbon oils wherein theoil is cracked in the presence of a finely divided catalyst which iscontinuously circulated through a cracking zone and a regenerating zonewhich comprises maintaining the cracking zone at a substantial elevationabove said regenerating zone, maintaining a layer of finely dividedcracking catalyst within said cracking zone, maintaining a second layerof finely divided cracking catalyst within said regenerating zone,passing an oxidizing gas upwardly through said regenerating zone at avelocity controlled to maintain a layer of said cracking catalyst in thebottom portion thereof, continuously downwardly withdrawing finelydivided catalytic material from the bottom portion of said regeneratingzone, discharging the catalytic material so withdrawn at a controlledrate into a stream of hydrocarbon vapors to be cracked, transferring thestream of hydrocarbon vapors containing said catalyst into saidconversion zone, controlling the velocity of hydrocarbon vapors passingupwardly through said cracking zone to maintain a dense fluidized layerof catalytic material therein, maintaining a confined vertical column ofsaid conversion catalyst between said cracking and regenerating,

zones, the upper end of said-column being in the back pressure on saidregenerating zone "in responseto the diiierence in pressure between saidcracking zone and said regenerating zone. 9. An apparatus'for treatinggases and solids which comprises an enlarged vertical vessel adapted tocontain a fluidized layer of finely divided solids, means forintroducing gases to be treated into the bottom of said zone at a ratecontrolled to maintain a dense fluidized layer of solids therein, aconduit having an upper end in open communication with the bottomportion of said vessel and adapted to continuously withdraw finelydivided solids therefrom, a valve in 'saidconduit for controlling therate of withdrawal of said finely divided solids, separating meanspositioned at a substantial elevation above such vessel adapted toseparate finely divided solids from gas, a pipe connecting saidseperating means with said conduit and adapted to carry a suspension ofsolids withdrawn from said treating vessel to said separating means, avertical column having its lower end in open communication with theinterior of said treating vessel and its upper end in open communicationwith the interior of said separating device and adapted to transfersolid material from said separating means to said treating zone, saidlastnamed conduit being adapted to sustain said solid material in adense fluidized state therein, means for maintaining said solids withinsaid column in a freely flowing, fluidized state capable of generating afluid pressure, said lastnamed column being of a height 'suflicient togenerate a substantial hydrostatic pressure at the base thereof, meansfor controlling the back pressure on said treating vessel and meansresponsive to the pressure difference between said separating means andsaid treating vessel to control said pressure control means.

10, An apparatus adapted for tne catalytic conversion of hydrocarbonoils which comprises an enlarged vertical conversion vessel, an enlargedvertical regenerating vessel supported at an elevation substantiallydiiferent tnan said conversion vessel,each of said vessels adapted tocontain a layer of finely divided conversion catalyst therein, conduitmeans for continuously withdrawing a stream of finely divided conversioncatalyst from the lower of said vessels, a valve in said last-namedmeans, a pipe connecting said last-named conduit with said upper vesseland adapted to transport'any divided conversion catalyst from the bottomvessel into the upper vessel, a vertical conduit extending between saidupper vessel and said lower vessel having the upper end in opencommunication open communication'with said cracking zone and the bottomof said column being in open communication with said regenerating zone,maintaining an aerating gas in admixture with said conversion catalystwithin said column in an amount controlled to maintain said catalyst ina fluid state capable of generating a fluid pressure at the basethereof, said column being of a height suflicient to maintain asubstantial pressure diflerence between said cracking zone and saidregenerating zone, and controlling the back with the interior ofsaidupper vessel and the lower end in open communication with the interiorof said lower vessel. means for maintaining an aerating gas in admixturewith the conversion catalyst contained in' said last-named conduit in anamount controlled to maintain said conversion catalyst in a freelyflowing, fluid state capable of generating a hydrostatic pressure, the

' height of said lnst-numedoonduit b81118 mimcient pressure on saidregenerating zone to regulate' the flow of catalyst from said crackingzone to said regenerating zone through said column to maintain asubstantial pressure differential between the upper vessel and the lowervessel, means for maintaining a back pressure on said lower vessel,means responsive to the diflerence in pressure between the upper vesseland the lower vessel for controlling said mu HARVEY E. W. BURNSIDI.HENRY J. OGORZALY.

